Calcium phosphate is widely used as a feed supplement to animal diets in order to provide animals with required amounts of phosphorus. Unfortunately, the phosphate rock commonly used to prepare calcium phosphate can contain large quantities of fluorine, as much as 4 percent. The presence of this much fluorine in an animal feed is known to cause fluorosis in the animals and thus it must be removed at some stage during preparation of the calcium phosphate. To be acceptable as a "feed grade" calcium phosphate, the phosphorus to fluorine weight ratio (P/F) in the product must be 100 or greater, a standard hitherto accepted in the art.
A host of methods for producing defluorined calcium phosphate are known. Methods which produce volatile fluorine compounds are most undesirable because of the severe air pollution they can create. Other methods are not used commercially because of their relatively high cost in relation to an inexpensive product. A preferred defluorination process is inexpensive and ties up the fluorine in liquid or solid form for easy and safe disposal.
U.S. Pat. No. 3,151,936 teaches single step defluorination of acid solutions of phosphate rock by simply subjecting the solutions to thin-film drying. More specifically, phosphate rock is digested with phosphoric acid and a more volatile mineral acid such as HCl. The solution is then spray-dried at high temperatures to yield feed grade calcium phosphate containing less than 5 percent volatiles and preferably less than 2 percent volatiles. The gaseous effluent from the dryer is condensed, defluorinated, and then recycled for use in acidulating more rock.
The reference process, it is noted, is directed towards essentially total removal of fluorine during the drying step. To achieve this in feasible commercial equipment such as a spray drier, very high temperatures i.e. 400.degree.F. or higher, are ordinarily required. Such high temperatures are known to cause pyrolysis of calcium phosphate to pyrophosphates and metaphosphates, a form wherein phosphorus is less available to animals, this not only diminishes product yield but also produces a contaminated product.
When total defluorination is achieved in the drying step, the acid condensate naturally contains large amounts of fluorine. Recirculation of this acid to the rock digestors, without intervening defluorination, is not feasible since fluorine would soon accumulate in the digestors to a point where satisfactory defluorination in the subsequent drying step would no longer be achievable.
A further disadvantage when total defluorination is achieved during drying is the danger of fouling the equipment with SiO.sub.2 formed by the hydrolysis of SiF.sub.4.
Applicant has now discovered novel methods which employ spray drying and crystallization techniques whereby calcium phosphate can be essentially totally defluorinated without the use of very high drying temperatures and all the aforementioned difficulties associated with such high temperatures.
Moreover, with applicant's process, condensed acid streams from the drying step can be conveniently recycled, without intervening defluorination, for use in acidulating rock.
Applicant, in the course of carrying out his process, further produces novel calcium phosphate compositions which contain varying amounts of calcium chlorophosphate but which, nevertheless, have an available phosphorus content comparable to commercially available feed grade calcium phosphates thereby making such compositions eminently useful as animal feed supplements.
These and other advantages of this invention will be apparent from the following disclosure.